The present invention relates to ferromagnetic thin-film memories and, more particularly, to ferromagnetic thin-film memory cells in which information states of memory cells therein are determined through magnetoresistive properties of the thin-film cells.
Digital memories are used extensively in digital systems of many kinds including computers and computer system components, and digital signal processing system. Such memories can be advantageously based on the storage of digital bits as alternative states of magnetization in magnetic materials in each memory cell, particularly thin-film magnetic materials, resulting in memories which use less electrical power and do not lose information upon removals of such electrical power.
These memories may use ferromagnetic thin-film materials in the memory cells through which access to the information stored therein can be provided by either inductive sensing to determine the magnetization state, or by magnetoresistive sensing to make such a determination, sufficiently small memory cell sizes favoring magnetoresistive sensing because significantly more output signal is available. Such ferromagnetic thin-film memories may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the memory cells and the memory operating circuitry.
Ferromagnetic thin-film memory cells can be made very small and packed very closely together to achieve a significant density of information being stored, particularly when provided on a surface in a monolithic integrated circuit. The magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell as will be further described below.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin film memory cells can be ameliorated to a considerable extent by providing a memory cell based on a bit structure with an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides a significant "flux closure", i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in a cell to affecting primarily Just that cell. This is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin.
Often such a digital memory is constructed by having a number of bit structures connected in series at junctures to one another in an end-to-end fashion to form a sense line current path. A series of current straps, or wordlines, are often provided in an orthogonal layout to a series of connected bit structures, or sense lines, so that a strap crosses each of the bit structures between the junctures connected thereto. In a magnetoresistive memory, such straps or wordlines are used both in the entering of, and the sensing of, information in the bit structures. This can be done by using currents in the wordlines for setting, or for determining the existing, magnetization state of bit structures storing bits of digital information each in a cell in the memory. Examples of such devices are described in more detail in U.S. Pat. Nos. 4,731,757 to Daughton et al and 4,780,848 to Daughton et al, both of which are hereby incorporated herein by reference, and in an earlier filed co-pending application by J. M. Daughton and A. V. Pohm entitled "Offset Magnetoresistive Memory Structures" having Ser. No. 07/786,128 and now U.S. Pat. No. 5,251,170, also hereby incorporated herein by reference.
The ferromagnetic thin-film in a memory cell, most often formed as two film portions on either side of the intermediate layer, is typically formed of an alloy of nickel, cobalt and iron in proportions of approximately 60%, 20% and 20%, respectively. Usually, these proportions are chosen to strongly reduce or eliminate any magnetostrictive effects in the film, and to improve certain other properties of the film for its intended use. In some situations, other materials are added to the alloy in relatively small amounts to improve certain properties of the film.
Such films can be fabricated by vacuum deposition or other methods and, if done in the presence of a magnetic field oriented in a selected direction, the resulting ferromagnetic thin-film will exhibit uniaxial anisotropy with the easy axis parallel to the direction in which the magnetic field is oriented during fabrication (which is typically such as to be parallel to the device wordline and so perpendicular to the path taken by the end-to-end bit structures forming a sense line). Because very large demagnetization fields would otherwise result, the magnetization vector of such a film will always lie in a plane of the film.
Furthermore, in accord with thermodynamics, the magnetization in such a film will arrange itself to minimize the magnetic energy therein. In the absence of any externally applied magnetic fields, such minimization occurs when the magnetization vector of a film portion parallels the easy axis of the film portion pointing in either direction along such axis.
However, the situation of such a film portion changes in the presence of externally applied magnetic fields and the minimization of magnetic energy may then occur with the magnetization vector of the film oriented at an angle with respect to the easy axis. As long as the magnetization of the film portion is in a single domain state, the magnetization vector can be caused to rotate with respect to the easy axis to reach angles determined by the externally applied fields, and this can occur without substantially affecting the magnitude of the magnetization.
Such ferromagnetic thin-films as those just described as the memory film portions on either side of the intermediate layer in a bit structure further exhibit magnetoresistance. Differences in direction between that of the magnetization vector in the memory film, and that of a current passed through the film, leads to differences in the effective electrical resistance in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the film and the current direction are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. Electrical resistance of such a magnetoresistive memory film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance follows a square of the cosine of that angle.
As a result, external magnetic fields can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of the film, and can vary it to such an extent as to cause switching the magnetization vector between two stable states which occur as magnetizations in opposite directions along the easy axis. Further, the state of the magnetization vector in such a film portion can be measured or sensed by the change in resistance encountered by current directed through this film portion. This provides the basis for a memory film portion to serve as part of a bit structure in a memory cell, the state of which is subject to being determined by effects occurring in currents applied to this portion. U.S. Pat. Nos. 4,829,476 to DuPuis et al and 5,012,444 to Hurst et al describe devices using such magnetoresistive sensing and are hereby incorporated herein by reference.
Although the two states in which the magnetization vector can occur in the bit structure ferromagnetic thin-films in the absence of external magnetic fields, i.e. the film magnetization being oriented in one of two directions along the film easy, axis were described as stable in the foregoing paragraph, such stability can be lost as the size of the memory cell, or the bit structure forming that memory cell, is decreased. This potential loss of stability comes about because the film magnetization is subject to being altered in direction from either of the two expected directions along the bit structure ferromagnetic thin-film easy axis even though no significant external magnetic fields are present. Such a situation in the film material can lead to the magnetization in some portions thereof being at an angle with respect to the easy axis, even to the point of being perpendicular thereto, or oriented at even a greater angle than 90.degree. thereto.
The primary such magnetic effect in the magnetoresistive, anisotropic ferromagnetic thin-films used in the memory cell bit structures is known as "curling" and results from the presence of large demagnetization fields in the bit structure memory film. The upper one of the two such memory film portions for a typical bit structure is shown in representational form in FIG. 1, and has shown thereon arrows giving the direction of the magnetization of that film at the locations at which those arrows begin. The easy axis in the presentation is oriented vertically in FIG. 1 and so substantially parallels the magnetizations occurring at the center of the film.
However, because of the relative closeness of the top and bottom edges of the film in FIG. 1, the demagnetizing fields due to "free poles" occurring along those edges are quite high and lead to reorienting the magnetization near those edges where the demagnetizing fields are the greatest, i.e. "curling" results. That is, the electron spins of the film material atoms at the top and bottom edges of the films are constrained to lie nearly parallel to these edges in the direction of elongation of the film portion, and so nearly parallel to the film hard axis, or nearly perpendicular to the film easy axis. This orientation results in minimizing the magnetic energy in the film due to the presence of both the demagnetization field and the anisotropy field which is a measure of the strength of the preference of the film material for the magnetization being along the easy axis.
The directions of the film material atoms electron spins, or the film magnetization, only gradually turn to parallel the film easy axis as the interior of the film portion is approached since the demagnetizing field decreases toward the film interior to the point of being more and more overcome by the anisotropy field. Thus, the magnetization component along the easy axis of the film portion in FIG. 1 has a value near zero at the upper and lower edges in FIG. 1 where the wordline over the bit structure would cross those edges, and gradually this easy axis magnetization component increases toward the interior of the film portion. In those interior regions of the film, beginning from near the upper and lower edges and extending to the film center, the easy axis magnetization component value increases toward the saturation value of the film material and reaches that value toward the film center in a single magnetic domain situation.
As can be seen, such magnetoresistive, anisotropic ferromagnetic thin-films, having their easy axis extending in a direction parallel to the wordline and perpendicular to the direction of elongation of the film, do not really saturate across the film along the easy axis, but only saturate across a portion of the film in the interior thereof. Further, while external magnetic fields may cause the magnetization of the interior portions to switch between opposite states along the easy axis, such switching will not have much effect on the magnetization of the portions near the upper and lower edges of the film portion in FIG. 1 because of the strength of the demagnetizing fields unless impractically large external magnetic fields are used in the switching arrangement described in the earlier cited U.S. Pat. Nos. 4,829,476 and 5,012,444 with conventional cell structure materials. Hence, the change in resistance measured by an electrical current along the direction of elongation of the film portion will not have a contribution to it from the upper and lower edge portions of the film of FIG. 1. This means there will be a smaller resistance than otherwise would be possible from a film portion of that size, and so a smaller output signal from the memory cell using such a film portion to indicate its magnetization state, and so the value of the digital bits stored therein. The switching arrangements of the last mentioned patents typically yield output signals based on only five to 35 percent of the possible magnetoresistive effect in a film portion of a memory cell.
The distance from the upper and lower edges of the film portion in FIG. 1 to the points where the film magnetization direction is more or less parallel to the easy axis, or its natural lowest energy direction in the absence of external magnetic fields, is found by minimizing the magnetic energy in the film. This includes the anisotropy energy, the exchange energy and the demagnetization energy. The anisotropy energy is minimized by a rapid return along this distance of the magnetization to being parallel with the easy axis. The exchange energy, determined from quantum mechanics and based on the change of angle between adjacent atoms magnetic spin, is minimized by having the adjacent electron spins parallel with one another and so tends toward having the magnetization parallel the easy axis along this distance but relatively weakly. The demagnetizing energy does not change very significantly with changes in the local magnetization, and so distances from the edges to the points where the magnetization parallels the easy axis is primarily determined from Jointly minimizing the anisotropy and exchange energies. Such distances are denominated as "curling distances."
Reducing the size of the memory film portion in order to construct memories having larger numbers of memory cells therein by increasing the surface density of such cells will further reduce the output signal from individual memory cells on the surface using the sensing methods of U.S. Pat. Nos. 4,829,476 and 5,012,444. This follows because any closer approach of the upper and lower edges tends to strengthen the magnetizing field due to the free poles therealong so that the "curled portions" near the edges remain with the magnetization thereof being urged toward being parallel to those edges over more or less the same "curling distances" in the film of smaller cells as it is in larger cells. Thus, the memory cell bit structure output signal generating portions, the film interior portions, effectively suffer the primary size reduction as the overall film surface dimensions are reduced in a memory cell.
Clearly, this output signal loss cannot be taken too far if an output signal is to be obtained from the memory cell portion that is sufficient to indicate the magnetization state therein with respect to the easy axis, and so minimum memory cell dimensions must be observed in these circumstances if an output signal of sufficient magnitude is to be obtained. Otherwise, if the cell size reduction is taken too far, there will be no portions of the film magnetization which would parallel the easy axis thereof, and so there will not be a pair of stable states along the easy axis for the magnetization to be switched between by external magnetic fields. Such result is shown in FIG. 2 where any vertical, or easy axis, magnetization component anywhere in the film portion is relatively quite small. Hence, a magnetization state switching and sensing arrangement is desired which will provide sufficiently greater memory cell output signals to indicate magnetization states occurring therein to permit use of a memory cell structure having reduced dimensions to increase the memory system digital bit, or stored information, density.